1. Field of the Invention
The present invention relates to a method for producing hardwearing and shock-resistant extrudates of ultra-high molecular weight polyethylene having an intrinsic viscosity of from about 800 to about 3,500 cm.sup.3 /g and to an apparatus for carrying out this method.
2. Description of the Related Art
EP-A-0 190 878 discloses that ultra-high molecular weight polyethylene (UHMW-pE) can be processed on single-screw extruders. Polyethylene powder is conveyed by means of an extruder screw, melted, and discharged via a die having a length/diameter ratio of .gtoreq.10. In order to obtain finished components having the desired properties, the extrudates are taken off and either remain unoriented or are oriented in a ratio of up to 30:1.
The conveying effect of the extruders is generally determined by the ratio of the frictional forces at the barrel to those at the extruder screw. It is true that attempts are made to counteract the resulting limits of the conveying effect by providing the inner wall of the barrel with grooves. However, these lose their effect in practical use as soon as shear stress in the grooves results in the generation of so much heat that the grooves become clogged with fused plastic particles. The result is thermal degradation of the material or a restriction of the extruder throughput to extremely low transport rates. The energy of fusion is preferably introduced via heat conduction, and it is for this reason that only a low level of output can be achieved and only thin extrudates can be produced.